Real time vehicle-to-vehicle and vehicle-to-infrastructure ad-hoc communication networks are known. A vehicle may be equipped with radio frequency (RF) transmission and reception capabilities. Transmitting and receiving vehicles are also referred to as “stations”. The IEEE802.11p standard provides a common platform for communications in such networks. All vehicles equipped with vehicle-to-vehicle communication equipment transmit periodically their location information in packets called “beacons”. However, IEEE802.11p cannot guarantee communications performance in a congested environment, since the deterministic arrival of beacons is not guaranteed. This leads to various problems. For example, lost safety messages translate into increased safety message “blind time”, which diminishes the safety benefit of vehicle-to-vehicle and vehicle-to-infrastructure communications.
With no access point to coordinate packets transmission, IEEE802.11p stations use a Communication Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism for transmission. With many surrounding vehicles on a highway or at an intersection, the hidden node problem becomes the limiting factor of the network. This problem is shown in FIG. 1, which illustrates a typical scenario on a road 110. A circle surrounding a vehicle represents its transmission range. A perfect path exists between a vehicle 100 and a vehicle 102. Vehicle 100 is “hidden” to vehicle 104. Therefore, vehicle 104 may initiate transmission despite of an ongoing transmission by vehicle 100. The concurrent (with that of vehicle 100) transmission by vehicle 104 will cause vehicle 102 not to “hear” vehicle 100.
An intersection such as intersection 220 in FIG. 2 between two roads 222 and 224 is even more problematic. Here, the hidden node problem is amplified due to lack of a line of sight between different vehicle pairs. As illustrated in FIG. 2, in a worst case, the transmission of a vehicle 200 to a vehicle 210 may be adversely affected or even interrupted by four different vehicles, 202, 204, 206 and 208.
Existing solutions to the hidden node problem include use of Unicast networks endowed with a Request to Send/Clear to Send (RTS/CTS) scheme. A “source” station that wishes to transmit requests permission from a “destination” station by sending a RTS message to that station. The destination station replies with a CTS message. This exchange constitutes a RTS/CTS “handshake”. Once permission is granted through the CTS message reply, the source station may transmit toward the destination station, with the guarantee that other stations surrounding the destination station are silent. In the past, the RTS/CTS scheme was not permitted use with broadcast packets. Now, standard revision 802.11e permits such use, albeit in an unspecified way.
Selecting an arbitrary vehicle for a RTS/CTS handshake for the purpose of broadcast transmission cannot work, because the arbitrary selection cannot clear a specific area. This can be seen in FIG. 1, in the exemplary case where vehicle 106 is selected for the handshake by vehicle 100, which sends the RTS message. Vehicle 106 is located close to vehicle 100. However, the returning CTS message does not propagate to a larger distance than the original RTS message, and therefore the clearance range is not extended. The problem is even clearer when one looks at FIG. 2. Therefore, arbitrary RTS/CTS cannot be applied in broadcast.
Another method promoted in the industry is time division multiple access (TDMA). By synchronizing all stations, each station is assigned a known time slot. It is guaranteed that during this time slot, no other station in proximity will transmit. Several TDMA concepts were studied in the context relevant herein. However, IEEE802.11p does not include TDMA, and it cannot be adapted or changed to include TDMA.